Glass articles



Amiga 1968 J, w, MORRls 3,396,075

GLASS ARTICLES Original Filed Sept. 17, 1962 2 Sheets-Sheet 1 INVENTOR.J OHN Hf. MO Q K15 if i ATTOKI EP' J. W. MORRIS Aug 6, 1968 GLASSARTICLES 2 Sheets-Sheet 2 Ezoau. s: :Ewa 20 n ENTOR. JOHN w. Maze/s m fb t N $3M 1 U2 all fig li llkfu ii. 1L9 I II I \G m u 3; zndvws ssv-us=10 svms 00123.5 0N cm i so sa'lmg v ATIOK/ EX United States Patent3,396,075 GLASS ARTICLES John W. Morris, Tarenturn, Pa, assignor toPittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation ofPennsylvania Continuation of abandoned application Ser. No. 224,894,Sept. 17, 11962. This application May 19, 1966, Ser. No.

6 Claims. (Cl. 16ll-199) ABSTRACT OF THE DTSCILQSURE The presentapplication is a continuation application of US. Ser. No. 224,894 filedSept. 17, 1962. and now abandoned.

According to the present invention, increases in impact resistance,breaking stress, penetration resistance, etc., are secured in glassarticles, particularly lime-soda-silica fiat glass articles such asWindshields spandrels, and other glass sheets suitable for use a viewingor like glass closures or partitions for buildings or transportationcompartments, by contacting the glass article with a potassium salt at ahigh temperature preferably above 875 F. for a brief period of time. Thecontact treating time at temperatures above 875 F. rarely exceeds 25 to40 minutes, but is longer for lower temperatures of treatment.

The contacting operation is conveniently effected after the conventionalproduction of the glass article has taken place, viz., after the glasshas been shaped into the desired article form. Advantageously thepotassium salt employed to contact the glass surface is potassiumnit-rate, and the contact or at least a portion thereof convenientlytakes place by immersing the glass article into a molten potassiumnitrate bath.

To some extent at least, the present invention involves an exchangewherein potassium from the potassium salt is introduced into the glasssurface apparently being exchanged for sodium present in the exteriorportions of the glass article. It is believed that this is an ionexchange phenomenon, wherein potassium ions are exchanged for sodiumions.

As a consequence of this treatment a compressive stress is establishedin the glass not only at the outer most surfaces of the glass article,but also extending from the surface for a finite thickness inwardlytowards the center of the treated glass article. While the centralregion of the glass sheet is under some tension, the magnitude of suchtensile stress, in pounds per square inch, is substanitally less thanthe maximum compressive stress, also measured in pounds per square inch,at the outer surface of the treated glass sheet.

The tensile stress in the central region is Well below the stress atwhich a notched piece of such glass fails under tensile stress, and thecenter tensile stress seldom exceeds 100 to 300 pounds per square inchfor glass thicknesses of 0.06 inch and greater, the center tensilestress being less for thicker glass samples. The ratio of maximumcompressive stress in the outer region(s) to maximum tensile stress inthe central interior region generally ranges from at least about 100 to500 to 1 for glass thicknesses of 0.06 inch and greater.

While the present invention will be illustrated hereinafter bydiscussion primarily relating to monolithic glass sheets and laminateshaving excellent strength, it should be realized that the basic effectof practice of the present invention is to increase substantially thescope of utility of glass to include its use Where high strengthproperties and surface compression properties are advantageous in amyriad of fabricated articles of commerce. Hence, the value of thepresent invention extends not only to Windshields but also to otherglass articles such as those used in the construction field and all ofthe fields where materials are required to possess high strengthproperties, e.-g., spandrels, windows, etc.

The invention is applicable to a wide range of sodiumcontaining glasses.In such glasses SiO B 0 and/0r A1 0 may be the principal network formersand various alkaline earth metal oxides may be present as fluxes to aidin the melting of the glasses. For example, silicate glasses containingin excess of 40 percent by Weight of SiO 0 to 15 percent by weight of B0 0 to 15 percent by weight of A1 0 0 to 25 percent by weight of CaO,MgO, SrO, BaO, PbO and/or ZnO and combinations thereof, 0 to 10 percentTiO 0 to 10 percent K 0 and 2 to 20 percent by weight of sodium oxidecan be employed in the practice of the invention.

The articles of the present invention offer dramatic improvements inbreaking strength and impact resistance when compared to conventionalglass articles. The articles of this invention are produced from windowand plate glass compositions having an initial sodium to potassiumweight ratio (prior to treatment) in excess of 1 to 1, and preferably inexcess of 5 to 1, for example, such glasses as soda-lime-silica glass.Such glasses usually have the following composition:

Percent by weight Na O 10 to 15 K 0 0 to 5 CaO 5 to 15 SiO 65 to MgO 0to 10 B203 0 t0 5 A typical soda-lime-silica glass suitable for use inac cordance with this invention has the following composition:

Percent by weight SiO 71.38 (usual variation 71 to 74%). N320 12.76(usual variation 12 to 14%). K 0 0.03 (usual variation 0 to 1%). C2109.67 (usual variation 8 to 12%). Mgo 4.33 (usual variation 2 to 5%). NaSO 0.75 (usual variation 0.1 to 1.0%). Fe O 0.15 (usual variation 0.1 to1.0%). A1 0 0.81 (usual variation 0.1 to 1.0%).

As shown by the above table, these glass compositions usually have a lare excess of sodium over potassium. The weight ratio of concentration ofsodium to potassium in such soda-lime-silica glass compositionsgenerally ranges from about 25 to 1 to upwards of to 1 and even highersince some soda-lime-silica glass compositions contain only traceamounts of potassium or no potassium. As a general rule, the higher thesodium content in the glass to be treated, the greater the strengthwhich can be obtained in the glass articles.

After treating soda-lime-silica glass according to the presentinvention, the chemical nature of the alkali oxide constituents of theouter surface of the glass article is altered radically. That is to saythat at the outermost surface of the glass the sodium is essentiallyreplaced by potassium. Yet at the central interior regions of the glassarticle the sodium content remains substantially unchanged by thetreatment. Therefore, at the outer surface(s) of the glass article,there exists a high potassium to sodium weight ratio, whereas in thecentral interior region(s) there exists a high weight ratio of sodium topotassium.

The region of maximum potassium concentration exists in a layer which isparallel to the surface of the glass and which extends for about 0.5 to1.0 micron in from the surface of the glass. The potassium concentrationtapers off gradually in any layer parallel to the surface of the glassas the distance of that layer from the surface of the glass is greater,with the potassium concentration ultimately being substantially the sameas in the untreated glass in a layer remote from the surface of theglass, i.e., 6 to microns from the surface of the glass. This is showngraphically in FIG. 3 and explained further in the description of FIG. 3hereinafter.

In the practice of this invention the potassium salt is contacted withthe glass article while the potassium salt is maintained in moltencondition. The equilibrium temperature of reaction is generallymaintained at a temperature above 875 F. to insure as rapid a treatmentcycle as possible thus enabling more articles to be produced in anygiven period of time.

As used herein the terms equilibrium temperature of reaction, contacttemperature, etc., are employed to denote the temperature at which thepotassium exchange is conducted. Conveniently this temperature isarrived at by (1) preheating and maintaining the potassium salt at atemperature preferably above 875 F., and (2) preheating the glass to atemperature preferably above 875 F. prior to contact of the glass withthe potassium salt, viz, preheating the glass to a temperatureapproximating that at which the salt is maintained.

When an immersion contacting technique is used, the temperature of thesurfaces of the glass sheets being treated is generally closelycorrelated to the temperature at which the potassium salt treating bathis maintained. Hence, it is generally preferable to preheat the surfacesof the glass sheets to be treated to a temperature approximating that atwhich the molten potassium salt bath is maintained prior to contactingthe glass sheets with the treating bath. However, it will be realizedthat the glass can be heated to a higher temperature than that at whichthe potassium salt bath is maintained, and the converse is also true.That is to say, the potassium salt bath can be maintained at atemperature below 875 F. as long as the glass is heated to asufiiciently high temperature to provide a composite or interfacialreaction temperature of 875 F. or higher. Conversely the glasstemperature can be below 875 F. as long as the potassium salt bath isheated to a sufficiently high temperature to insure a reactiontemperature of 875 F. or higher. At significantly lower temperatures,the potassium exchange procedure is much slower.

Thus, it is within the purview of the present invention to employ anycombination of glass temperature and potassium salt temperature whichwill yield a composite (equilibrium) temperature of reaction of about875 F. and above. Further it is within the purview of this invention touse potassium salts other than potassium nitrate, such as potassiumchloride, sulfate and mixtures thereof, which can be molten or solid atthe contact temperature.

At lower temperatures the effect of such contact is much slower with theresult that production of glass articles herein contemplated isdifficult to achieve within periods of time which are commerciallypracticable. For example, one hours immersion of lime-soda-silica glassin molten potassium nitrate at 700 F. does not improve the strengthproperties of the glass substantially. Much longer periods of immersionat this temperature are required to produce strengths comparable to thatachieved in the minimal time periods, e.g., 5 to 10 minutes at thehigher temperatures. At temperatures exceeding 875 F., the desiredstrength improvement is attained even more rapidly.

The upper limit of the contact temperature is dependcut upon thesoftening temperature and melting temperature of the glass article beingtreated. Thus, the contact temperature cannot be so high as to exceedthe melting temperature of the glass composition, but it can exceed thestrain point and even the softening point of the glass composition undercertain circumstances.

Thus, as long as the glass can be properly supported, the contacttemperature can even be maintained at a temperature above the softeningtemperature of the glass provided that the contact treatment timeat'these elevated temperatures is of sufficiently short duration toavoid thermal relaxation of the potassium exchange induced strengthcharacteristics. In fact, in some cases it is possible to maintain thecontact temperature within the softening temperature range of theparticular glass article being treated. Under these thermal conditionsextremely short contact times can be employed, viz, contact times of theorder of one minute, and even less.

While temperatures as high as 1200 F. to 1400 F. can be employed, asmentioned above, consistently superior results are secured using contacttemperatures ranging from about 900 F. to 1100 F.; and for most glassarticles, this temperature range is sufficiently high to secure thecomposite strength characteristics in the improved glass articles of thepresent invention.

.The contact treating time, viz, the length of time during which theglass article is contacted with the potassium treating salt, generallyranges from about 5 to about 25 to 40 minutes at the high temperaturesmentioned here- 'inabove. In general, treatment durations substantiallyin excess of 25 to 40 minutes at temperatures substantially in excess of875 F. do not materially improve the strength properties which attendthe articles of the present invention over those obtained in shorterperiods. In fact, pro longed contact at these temperatures can beobjectionable. Thus, it has been observed that long heating (two or morehours) of the treated glass at temperatures substantially in excess of900 F., whether while in contact with potassium nitrate or otherwise,results in loss in strength properties. For example, heating the glasssheet for a period in excess of 2 hours at 1000 F. while it is incontact with the potassium results in a material loss in the strengthimprovement attained at the shorter periods mentioned above. The reasonfor this loss is not completely understood; however, it can bepostulated that over extended periods of time at these high temperaturesthere is a relaxation of the compressive stresses set up in the glass.Also there may occur in the glass a rearrangement or migration of thepotassium and sodium, which rearrangement or migration results inreduced strength. For these reasons the glass, after treatment, shouldbe cooled below 875 F., preferably below 300 F. to 500 F., rapidly andin any event before the compresive stress imparted to the glass ismaterially reduced. However, cooling should not be carried out sorapidly as to break the potassium treated glass due to thermal shock.

Contacting the glass with the potassium treating salt for timessubstantially less than 5 minutes can be satisfactorily conductedprovided that sufliciently high temperatures are employed to secure thenecessary potassium exchange in the surface regions of the glass articlebeing treated, and provided that the potassium exchange (p0- tassiumdiffusion) is conducted into a suflicient depth of the outer glasssurface to insure a resulting potassium to sodium ratio after treatmentsuch that the exchange induced surface ratio concentration of potassiumto sodium is in excess of 1 to l for a surface depth of at least about 1micron and preferably even deeper.

That is to say that in order to secure the utmost benefits of thestrength characteristics which can be impart-ed to the improved glassarticles of the present invention, it is necessary to conduct thepotassium exchange so that there is a depth penetration of potassium forat least a finite thickness towards the mid-plane of the glass article.Thus, the increase in strength is sufficiently deep on a penetrationlevel so that subsequent abrasive treatments, such as those involved byhandling in prepressing and autoclaving to form safety-glass laminates,or other fabricating operations to which the treated glass sheet(s) maybe later subjected, will not cause substantial loss of the strengthcharacteristics imparted by the potassium exchange induced treatment.

Another factor to be considered when lower contact times, viz, contacttreating times substantially below 5 minutes, are employed is the effectthe higher treating temperatures can have the viscosity characteristicsof the glass article being treated. Where avoidance of thermaldeformation of the glass article is a significant factor, temperaturessubstantially below the melting temperature of the particular glasscomposition should be used. The terms melting temperature, strain point,and softening point as used herein are standard terms in the glass artand are defined in the American Society for Testing Materials Booklet(1162-56. However, even in view of the above criteria, it is possible,when using a glass article supporting system which can minimize orsubstantially eliminate the effects of thermal deformation, to employtreating temperatures above that at which the glass composition exhibitssome plasticity. Thus, when glass supporting systems, e.g., a partial ortotal gas support (wherein the supporting medium is air or an inertgaseous substance) are employed in conjunction with the use of elevatedtemperatures, the contact times can be reduced to a contact treatmenttime even as low as several seconds to several minutes.

In such a case, the contacting procedure whereby the glass article iscontacted with the molten potassium treating salt can be convenientlyeffected by some means other than immersion. For example, the potassiumsalt can be deposited readily on the glass surface prior to raising thetemperature of both the glass and the potassium salt to the elevatedtemperatures required for treating. Or the potassium salt can bedeposited from above by flowing it onto the outer surface of the glasswhile both the glass and potassium salt are maintained at treatmenttemperatures.

It is also possible to employ a contacting procedure which involves acombination of immersion and nonimmersion procedures with the formeroccurring prior to the latter. In such a procedure the immersion (intank) contact can be at very high temperatures, for example, l00O F. toll00 F. for an extremely short period of time, for example, two minutesor less, followed by nonimmersion (out of tank) contact at lowertemperatures, for example, 900 F. to 975 F., for longer periods of time.The total contact at the temperatures above 875 F. is, however,generally within a range previously discussed. For example, the glassarticle is preheated to a temperature of approximately 900 F. to 950 F.and then dipped into a preheated molten potassium salt bath maintainedat about 1050 F. for a period of from to seconds to initiate potassiumexchange. The glass article is removed from the salt bath with a saltfilm being retained on its surface and subjected to a temperature of 925F. for ten minutes to allow further out of tank exchange to take place.The article is then cooled. As used herein the terms contact time,contact treating time, etc., refer to the total period of time duringwhich the glass surfaces are in contact with the potassium treating saltat a temperature of at least 875 F. Other variations in techniques ofachieving contact of the glass article with the potassium treating saltwill be apparent to those skilled in the art once the benefit ofapplicants invention has become known.

The benefits and advantages attendant to the articles of the presentinvention are generally applicable to glass articles regardless of theirthickness. Thus, for example, dramatic increases in breaking strengthare secured in glass windshield lights which are of a thickness of about0.090 inch and inch. However, thinner or thicker windshield lights canalso be treated with an attendant increase in strength of these glasswindshield lights. Therefore, the present invention includes glassarticles irrespective of their thickness. Hence, the present inventionis adaptable to increasing the surface strength characteristics of verythin and very thick glass articles, 'viz., glass articles havingthickness of the order ranging from 5 of an inch to inch all the way upto extremely thick glass articles such as structural glass articles,e.g., glass doors.

The nature of the potassium salt which is employed to treat the glassarticles is important in that a potassium salt must be used which can beemployed at high temperatures, i.e., temperatures ranging from about 875F. to 1100 F. or even higher, without objectionable decomposition of thepotassium salt occurring. The potassium salt of choice is potassiumnitrate. The potassium nitrate salt can be employed either alone or inconjunction with other potassium salts, e.g., potassium chloride, toconstitute the potassium salt treating bath to etfect the exchange ofpotassium for the sodium present in the glass article. When a mixedpotassium salt bath is employed, such as a mixture of potassium nitrateand potassium chloride, it is preferable to employ a predeterminatingmole percent of potassium nitrate. An exemplary mixed potassium salttreatin bath within the purview of the present invention is one havingabout 70 mole percent potassium nitrate and 30 mole percent potassiumchloride. However, the advantages attendant to the method of the presentinvention can be secured using a potassium nitrate-potassium chloridetreating bath having a potassium nitrate mole percent ranging from about50 percent to percent.

In the production of the articles of this invention over extendedperiods of time, when a plurality of glass sheets are successivelyimmersed in the molten potassium salt bath, sodium gradually accumulatesin the bath from day to day or from week to week. For example, in atypical instance it was noted that seriatim dips of 133 pairs ofwindshield lights resulted in an increase of sodium in the potassiumnitrate bath from 0.073 percent to 0.095 percent by weight.

As the sodium content increases, the degree of compressive stress in theglass surface falls causing a corresponding reduction in glass strength.Thus, unless proper precautions are observed, sheets which are dipped ina late stage of a campaign are not as strong as sheets dipped in theearly stage thereof because of sodium accumulation in the potassium salttreating bath.

In general, the sodium content of the bath is held below 10 percent byweight, and preferably below 5 percent by weight, based upon thecombined weight of sodium and potassium in the bath.

At the onset and during the early stage(s) of any given dippingcampaign, the sodium content is generally below 2 percent by weight andrarely exceeds 1 percent by weight. Most preferably the sodium contentranges downwardly from 1 percent by weight to a value approaching andeven reaching 0 percent by weight.

The sodium content of the dipping bath should not be permitted to varymore than 5, and preferably less than 2 percent by weight (based uponthe total combined weight of sodium and potassium in the molten bath)from the early (low sodium content) stage of dipping to the later(higher sodium content) stage thereof even though pluralities of glasssheets are dipped over a period of 1 to 20 weeks. This control can beachieved in several ways. For example, the dipped sheet may be withdrawnrapidly and allowed to drain outside the bath so that the drippings(which can contain sodium) are not returned to the bath. Moreover, inthis case the period of immersion can be held to a minimum so that mostof the sodium exchange takes place after the glass is withdrawn from thebath. In addition, the bath composition can be adjusted by addition ofpotassium salt in amounts sufficient to replace consumed or withdrawnpotassium. Also portions of the bath may be withdrawn and purified.

The invention will be further understood with reference to theaccompanying drawings in which:

FIGURE 1 is a schematic plan view of a windshield;

FIGURE 2 is a cross-sectional view of the windshield of FIGURE 1 takenalong the line IIII; and

FIGURE 3 is a graph illustrating a typical variation of the potassiumconcentration for given penetration depths in a glass treated accordingto the present invention.

Referring now more particularly to FIGURES 1 and 2, there is shown acurved laminated windshield W having matching curved glass lights 1 and2 with interior facing plastic contacting surfaces 3 and 4, exteriorfacing surfaces 5 and 6, and outer peripheral seamed edge surfaces 7 and8, respectively. All of the surfaces of both lights, including the outerperipheral edge surfaces, have been potassium salt treated. The interiorfacing surfaces 3 and 4 are tenaciously adhered together by a plasticinterlayer sheet 9 which can be composed of polyvinyl butyral or anysuitable thermoplastic, safety-glass interlayer material.

FIGURE 3 graphically represents the potassium concentration (triangularpoints) and sodium concentration (circular points) vs. penetration depthfor a 4 inch by 6 inch by .090 inch glass sample treated with a moltenpotassium nitrate salt in accordance with this invention. The sample wasa soda-lime-silica polished plate glass sample of Composition B as givenin Example I. The sample was preheated for about 17 minutes at atemperature of about 925 F. and then immersed into a molten potassiumnitrate bath maintained at 925 F. for an immersion period of minutes.The sample was then cooled gradually to room temperature prior toanalysis.

In conducting the analytical test, which provided the data for thecurves of FIGURE 3, the potassium treated sample was repeatedly dippedin 2 percent aqueous HF etching solution for about 2-minute dip periodsto remove about a one micron layer of glass for each of the ten dips.The sample was weighed before and after each of the ten dips todetermine the weight of each layer removed during each clip. Thethickness removed was also noted for each etching dip. After dipping forabout two minutes at each dip, the sample was removed from the etchingbath and rinsed with deionized water to stop the etching. The wash waterwas then added to the etching bath. Each etching bath sample was thendried, and subjected to spectrophotometric analysis to determine thepercent by weight of potassium and sodium, respectively, present in eachetched layer. Fresh 2 percent HF aqueous solutions were used for eachsuccessive dip. The data for the potassium and sodium curves of FIGURE 3is hereinbelow reproduced.

8 While the windshield depicted in FIGURE 2 of the drawings has all ofits surfaces potassium treated, it should be realized that the presentinvention also embraces windshields having only the following surfacesor at least selected portions thereof potassium exchange treated:

(a) both interior facing, plastic contacting surfaces 3 and 4;

(b) both exterior facing surfaces 5 and 6;

(c) any one or both of edge surfaces 7 and 8;

(d) interior facing surface 4 and exterior facing surface (f) one orboth of edge surfaces 7 and 8, in conjunction with any one of treatingpatterns (a), (b), (d) and (e);

(g) both interior facing surfaces 3 and 4 and exterior facing surface 5;

(h) both interior facing surfaces 3 and 4 and exterior facing surface 6;

(i) both exterior facing surfaces 5 and 6 interior facing surface 3;

(j) both exterior facing surfaces 5 and 6 and interior facing surface 4;

(k) one or both of edge surfaces 7 and 8, in conjunction with any one oftreating patterns (g) to (k), inclusive;

(1) any one of surfaces 3, 4, 5 and 6;

(m) one or both of edge surfaces 7 and 8, in conjunction with treatingpattern (1);

(n) all of surfaces 3, 4, 5 and 6; and

(0) any one or both of edge surfaces 7 and 8, in conjunction withtreating pattern (n).

Various advantages are secured with the various treating patterns setforth hereinabove. For example, according to treating pattern (1) onlyexterior facing surface 5 is potassium exchange treated. This providesstrength in the outermost (exposed) surface of the windshield to avoidbreakage due to stones or other small objects which may be tossed fromthe street onto the windshield by the tires of other cars on the road.

According to treating pattern (0) only both edge surfaces 7 and 8 can bepotassium treated. This provides the windshield with edge strength toresist breakage when the edges thereof are subjected to mechanicalshocks and vibrations transmitted from the chassis of the vehiclethrough the windshield mounting gasket into the windshield. Thetransmitted mechanical shocks and vibrations act first upon the edges ofthe windshield. Hence, increase in edge strength assists in preventingbreakage.

According to treat-ing pattern (a) only interior facing,plastic-contacting surfaces 3 and 4 can be potassium exchange treated.This provides protection for the treated surfaces 3 and 4 from abrasionand weathering, and hence insures retention of their strengthcharacteristics.

Of course, the greatest measure of strength benefits of the presentinvention are provided according to treating Weight of Thickness ofCumulative Penetration Potassium Gram moles Sodium Gram moles Etchingbath glass removed glass layer thickness of depth for content (pcrofpotassium content (perof sodium per sample (grams) removed glass removedplotting points cent by weight) per grams cent by weight) 100 grams of(microns) (microns) (microns) of samples sample The values for grammoles of potassium per 100 grams of glass sample were obtained bydividing the analyzed percent by weight of potassium for each givenetched sample by the atomic weight of potassium. The corre spondingvalues of sodium were calculated in the same manner except, of course,the atomic weight of sodium was used as the divisor.

pattern (0) wherein all of surfaces 3, 4, 5, 6, 7 and 8 are potassiumtreated. This is the case because all of the surfaces of both windshieldlights 1 and 2 are provided with increased strength.

The following examples serve to illustrate the invention in greaterdetail. However, it should be understood that the invention in itsbroadest aspects is not necessarily interior facing surface 3 andexterior facing surface limited to the particular materials,thicknesses, and process conditions set for the below in the examples:

Example I Thirteen 2 by 2' flat soda-lime-silica polished glass platesof the thicknesses and glass composition specified in Table 1 below hadeither of the following compositions:

Component Composition A Composition 13 (percent by weight) (percent byweight) The glass plates of Samples 1-9 were placed in horizontalstainless steel carrying racks with each sheet being carried at a slopeof degrees with the horizontal axis of the carrying rack. Fiber glasstape without binder was wrapped around the support posts of thestainless steel carrying racks and over the weight bearing supportpoints to reduce possible mechanical damage to the edge of the glasssheets. The fiber glass tape also served to reduce the heat transferrate from the edge of the glass to the support posts. Samples 1-9 wereconveyed with the width dimension (distance from bottom edge to topedge) parallel to the horizontal path of the racks along the treatmentline. Samples 1-9 were then preheated in a glass lehr to raise the glasstemperature from ambient room temperature to a temperature of from 900to 925 F. using radiant gas burners operated at an air-to-gas ratio of12 to 1. The preheated operation was conducted over a period of about 17minutes.

Then the preheated plates of Samples 1-9 were immersed with their racksinto a molten potassium nitrate bath maintained at a temperature of 925F. for an immcrsion period of 10 minutes.

The lehr was heated from above with gas heat supplied by radiant gasburners disposed several feet above the level of the molten pool, andthese burners were also operated at an air-to-gas ratio of 12 to 1, andfrom below by electric heating devices placed in the bottom of the lehrexternal to the treating tank to insure the maintenance of a uniformtreating temperature in the molten potassium nitrate bath. Thetemperature controls employed in the tank containing the moltenpotassium nitrate are such that allow the temperature of 925 F. to bemaintained within about a plus or minus 10 F. temperature deviation.

The tank containing the molten potassium nitrate was 6 X 3' x 13.5 toallow total immerision of the flat glass plates. The molten salt bath wamaintained at a 12" depth sufiicient to insure total immersion of theglass sheets of Samples 1-9.

After contacting the glass plates of Samples 1-9 with molten potassiumnitrate for a period of approximately 10 minutes, the racks were thenraised out of the molten potassium salt treating bath, and conveyedthrough an insulated tunnel cooling section of the lehr to allow coolingthereof in a gradual manner so that the drop in temperature during thecooling operation would not cause warpage or breakage.

After effecting a substantial degree of cooling, viz, cooling totemperature of approximately 200 F. to 250 F., the glass was thenallowed to cool to the ambient room temperature outside the oven. Theentire period of cooling from the time the glass was removed from themolten potassium salt pool was about to minutes.

The glass was then racked while at approximately room temperature withthe width dimension of the racked plates in the vertical position, andthe treated glass plates were cleaned to remove the excess potassiumnitrate. The washed plates were then allowed to dry in a verticalposition at room temperature. Water was employed for this purpose.

The four control samples, Samples 10-13, were not potassium treated, butwere of the same compositions as used in Samples l-9 as noted fromTable 1. Load strength test samples 4" x 4 were cut from all sixteenplates and subjected to load strength testing using concentric ringloading on the four-inch square test samples. The outer ring had adiameter of 3 inches and the inner ring had a diameter of 1.5 inches.The load speed was 0.2 inch per minute, and the reported load strengthsare the strengths (lbs) at which failure (glass breakage) occurred.

The average potassium surface penetration depth (microns) of Samples 1-9was determined by optical birefringence. This method is accurate fordetermination of qualitative potassium diffusion within a realm ofexperimental error of about two microns plus or minus. These values arereported in Table 1.

TABLE 1 Average load Average K Glass Sheet load strength surface Samplecomposition thickness strength penetration (inches) (pounds) (microns) A0.125 A 0. 1, 020 5. 5 A 0.125 B 0. 125 B 0. 125 1, 020 6. 0 B 0.125 B0. 090 B 0. 090 788 6 0 1g 0. 090 1 0. 125 B 0.125 I B 0. 090 B 0. 090 l293 As will be noted from the above data, the strength of the treatedglass articles of this invention is greater than that of conventionaluntreated polished plate glass.

Example II Eight 12" X 12" flat, polished, plate glass samples (Samples14-21) of Composition B, as given in EX- ample 1, were subjected topotassium salt treatment in a molten potassium nitrate bath under thesame conditions as set forth above in conjunction with Example I.

Samples 14-21 were then subjected to impact resistance tests using a 0.5pound, 1.5 inch diameter steel ball dropped from various heights tofailure of the glass samples (glass breakage).

Eight control samples (Samples 22-29) 12" by 12 flat, polished, plateglass also of Composition B were also tested. These samples were notpotassium treated, however. All samples were clamped in a support frameprior to testing.

The impact resistance data of Samples 14-29 is re produced hereinbelowin Table 2.

TABLE 2 Average drop Average contact SllQObtlllCknGSS height to velocityat impact Samp (inch) break (feet) to cause breakage (miles per hour) Aswill be noted from the above data, Samples 14-21, potassium treated inaccordance with this invention, were more resistant to impact than wereSamples 22-29 which had not been treated.

1 1 Example III Twenty-four soda-lime-silica glass windshield lights ofthe glass composition and thickness noted below in Table 3 were cut frompolished, flat glass sheets into matching pairs (doublets) for bending.

After cutting, the windshield lights were edge seamed by combineddiamond wheel and abrasive belt techniques. The lights were then washed.Then a separator (parting) material, viz, Celite, which is diatomaceousearth manufactured by the Johns-Manville Corporation, was dusted on theinterior facing, plastic-contacting surfaces of each matching light inpreparation for bending. The matching pairs of lights were thensubjected to preheating followed by bending on a bending mold at bendingtemperatures ranging from 1000 F. to 1100 F. for a total period,inclusive of preheating and bending, ranging from 15 to 18 minutes untilbending was completed.

The windshield lights were bent to the general complex curvature shownin FIGURES 1 and 2. The bent sheets were then annealed and graduallycooled to room temperature over a period of 27 to 30 minutes. The entirepreheating, bending, annealing and room temperature cooling cycle wasperformed in a period ranging from 42 to 48 minutes. After cooling, theparting material was allowed to remain on the glass sheets.

The doublet pairs, Samples 30-41, were then separated and placed in ahorizontal stainless steel carrying device (rack) with each individualsheet being carried at a slope of 15 degrees with the horizontal axis ofthe carrying rack.

Doublet Samples 30-3 5 were then preheated in the glass lehr to raisethe temperature of the glass from the ambient room temperatureconditions prior to contact with the potassium salt to a temperatureranging from 900 F. to 925 F. using radiant gas burners operated at anair-to-gas ratio of 12 to 1. This preheating operation was conductedover a period of about 17 minutes. Then the preheated doublet sheets ofSamples 30-35 were immersed with their racks into a molten potassiumnitrate bath maintained at a temperature of 925 F. for an immersionperiod of minutes under the same conditions as set forth above inExample I.

Doublet Samples 36-41 were neither treated with the potassium nitrate,nor subjected to any heat treatment except that involved due to theheating occurring during bending.

The twelve pairs of bent doublets, Samples 30-41, were then washed withwater to remove the parting material (Samples 30-41) as well as thesolidified potassium nitrate salt (Samples 30-35). The washed doubletamples were then racked in a vertical position and allowed to dry atroom temperature.

All twelve matched, bent doublet pairs were then laminated usingpolyvinyl butyral interlayer sheets of the thickness noted in Table 3below to produce safety glass Windshields.

The laminating was conducted in two stages. The first stage, viz,prepressing, was performed using rubber tubing edge channels asdescribed in Keim, U.S. Patent 2,948,645. -The matching pairs of bentlights were assembled with the interlayer therebetween to form sandwichstructures. The tubing was then fitted around the edge periphery of eachsandwich and connected to a vacuum source. The prepressing was performedat a temperature of 300 F. for 13 minutes using a vacuum of 26-29 inchesof mercury.

The prepressed windshield samples were then autoclaved in an oilautoclave for 45 minutes at 275 F. using 200 pounds per square inchautoclaving pressure. Following the autoclaving the laminatedWindshields were allowed to cool to room temperature. Table 3 below setsforth the glass sheet thickness, glass composition and interlayerthickness for each of laminated windshield 12 Samples 30-41. The glasscompositions noted are those given in Example I above.

TABLE 3 Glass sheet Potassium thickness salt treated (inch) Interlayerthickness 30. Composition B .do... Composition A- 41 .d0

Windshield Samples 30-41, inclusive, were then mounted in a standardrubber glazing channel retained in an angle iron frame forimpact-penetration testing using a lead-filled, wooden ball which was 8inches in diameter and weighed 30 pounds. The sample is considered tohave passed the test if the ball is retained by the sample 50 percent ofthe time for any given drop height.

When subjected to the above test, Windshield Samples 30-35 possessedsuperior impact-penetration resistance as compared to the untreated,conventional control windshields (Samples 36-41).

Example IV Thirty-six 4 inch by 4 inch by 0.125 inch polishedsoda-lime-silica glass plates of identical glass composition (Samples42-77) were preheated for ten minutes to the respective temperature atwhich the potassium salt bath was maintained and then immersed in amolten potassium nitrate bath for immersion periods noted below. Thenthe plates were removed from the potassium salt bath and heated whilestill in contact with the immersionprovided potassium nitrate films forthe heating periods noted.

Samples 42-47, inclusive (Group I), were preheated for 10 minutes at 950F. and then immersed in the molten potassium nitrate bath for a periodof 15 seconds. The potassium nitrate salt treating bath was maintainedat a temperature of 950 F. Then Samples 41-46 were heated at 950 F.while out of the treating tank for a period of approximately 15 minutes.

Samples 48-53 (Group II) were preheated for 10 minutes at 950 F., anddipped for 30 seconds at 950 F. This was followed by out of tank heatingat 950 F. for 15 minutes.

Samples 54-59 (Group III) were preheated for 10 minutes at 950 F., anddipped for 60 seconds at 950 F.

This was followed by out of tank heating at about 950 F. for 15 minutes.

Samples 60-65 (Group IV) were preheated for 10 minutes at 1050 F., anddipped for 15 seconds at 1050 F. This was followed by out of tankheating at 950 F. for 15 minutes.

Samples 66-71 (Group V) were preheated for 10 minutes at 1050 F., anddipped for 30 seconds at 1050 F. This was followed by out of tankheating at 950 F. for 15 minutes.

Samples 72-77 (Group VI) were preheated for 10' minutes at 1050 F., anddipped for 60 seconds at 1050 F. This was followed by out of tankheating at 950 F. for 15 minutes.

Control samples (Samples 78-113) of the same size, thickness, and glasscomposition as Samples 42-77, were subjected to the same temperatureconditions as Samples 41-76, respectively, but were not contacted withthe potassium nitrate treating bath.

Samples 42-113 were then strength tested for load strength using theconcentric ring test described in Example I above. Samples 42-77,respectively, possessed superior load strengths as compared to theconventional 13 polished soda-lime-silica glass plates (Samples 78-113,respectively) which were not subjected to potassium treatment inaccordance with this invention.

From this example, it is evident that the potassium treatment can beconducted using short periods of immersion at high temperatures whilestill securing superior strength characteristics as compared tountreated plates.

Example V Flat polished plate glass samples 12 inches by 12 inches by0.125 and 0.090 inch, respectively, were potassium treated in accordancewith the procedure set forth in Example I. These glass plates were allof Composition B as given in Example I.

The treated samples were then washed and laminated using 12 inch by 12inch polyvinyl butyral interlayer sheets 0.015 inch and 0.025 inchthick, respectively. The laminating procedure consisted of prepressingand autoclaving under the same conditions set forth above in Example IIIwitih the exception that no rubber edge channel was used.

Corresponding flat control sample plates were also laminated, but werenot subjected tothe potassium nitrate salt treatment.

Then all forty-eight laminate samples were tested for impact strength(drop height to breakage) with steel balls 0.5 inch, 0.687 inch and 0.5inch and weighing 7.6 grams, 21.7 grams and 220 grams, respectively. Theabove described impact tests were conducted at temperatures of 75 F.(clamped frame).

Also, all samples were subjected to impact strength tests which wereperformed using a 0.5 pound steel ball to determine drop height tobreakage. The 0.5 pound impact tests were conducted on all samples attemperatures of E, room temperature (68-78 F.), and 120 F.,respectively.

The glass sheet thickness, interlayer thickness, and testing conditionsare summarized below in Table 4 for the treated samples (Samples114-137).

The untreated corresponding control samples (Samples 13'8-161) were ofthe same glass and interlayer thickness, respectively, and weresubjected to the same tests, respectively, as noted in Table 4 for thetreated samples. Of course, Samples 138-161 were not treated withpotassium nitrate.

The impact strength of the potassium nitrate salt treated flatlaminates, Samples 114-137, was superior to that of the untreatedcontrol samples, Samples 138-161, under all test conditions.

14 inch by 12 inch by 0.090 and 0.125 inch, respectively, were potassiumtreated, washed, and laminated with 12 inch by 12 inch polyvinyl butyralinterlayer sheets 0.015 and 0.025 inch thick, respectively, as inExample V.

Corresponding fiat control sample sheets were also laminated, but werenot subjected to the potassium nitrate salt treatment.

Then all samples were subjected to an extensive series of high velocityimpact tests conducted to compare the damage done to the 12 inch squarecontrol laminates and the 12 inch square potassium nitrate treatedlaminates when hit with brass-coated steel B.B.s each weighing 0.351gram and of a 0.170 inch diameter.

The B.B.s were fired at the laminates from a Daisy air rifle at adistance of 15 feet from the laminates at a velocity of 187 miles perhour (275 feet per second). Five shots were made at spaced locations oneach of the laminates at a 90 degree angle for glass temperatures of 15F., F., 120 F. and 140 F. The same comparisons were repeated at anglesof 45 degrees and 15 degrees for glass temperatures of 15 F., 75 F. and120 F.

Additional tests were made using a whamo sling shot device at a 20 inchpull back at 75 F. to hurl (a) 0.250 inch steel balls weighing 1.1 gramsand (b) 1 inch by 0.5 inch road stones with an average weight of 11.0grams. Projectiles (a) and (b) were shot from the whamo sling shot at avelocity of 55 miles per hour feet per second) and a distance of 15 feetfrom the laminates. The angle of contact was degrees.

The laminated samples tested were of the four types noted in Table 5below, with the only difference being the presence or absence of thepotassium salt treatment.

TABLE 5 Laminate Glass sheet Interlayer type thickness thickness (inch)(inch) Example VII Four soda-lime-silica fiat polished plate glasssheets of Composition B in Example I and having dimensions TABLE 4 75 F.0.5 pound ball Glass sheet Potassium Interlayer Sample thickness salttreated thickness 0.5 inch 0.687 inch 1.5 inch Room (inch) (inch) 7.6gram 21.7 gram 220 gram 0 F temp. F.

B 1 Ball Ball Example VI Flat polished plate glass samples ofComposition B as set forth in Example I and having dimensions of 12 of32 inches by 4 inches by 0.125 inch were potassium salt treated as inExample I.

Four additional sheets of identical glass composition and havingdimensions of 32 inches by 4 inches by 0.250

inch were not potassium salt treated and served as control samples.Instead of the potassium salt treatment, the control samples werethermally tempered. The thermal tempering was performed by rapidlyraising the glass to a temperature of from 1150" F. to 1250 F. (meanpeak temperature of 1200 F.) in a period of from 3.5 to 5.5 minutesfollowed by rapid cooling, i.e., quenching for from 20 to 30 secondswith compressed air. The air prior to compression was at roomtemperature whereas after compression the air temperature at the time ofquenching ranged from 90 F. to 100 F. After quenching, these controlsamples were then allowed to further cool to room temperature. Thethermal tempering cycle averaged approximately six minutes for the foursamples.

All eight samples were then subjected to cutting tests using apunch-type cutting device to determine if the samples could be cutwithout shattering.

The glass sheets which were potassium salt treated did not shatter uponcutting, but the thermally tempered sheets shattered immediately uponcutting.

Then 4 inch by 4 inch load strength samples were cut from each potassiumsalt treated sheet, and these samples were subjected to concentric ringload testing as in Example I. The load strengths of these samplescompared closely to the reported average load strength for Samples 4-6of Example I.

Thus, the glass sheets treated in accordance with this invention can becut after treatment without loss of strength.

It was impossible to load strength test the thermally tempered samplesafter cutting due to their shattering.

The potassium treated glass articles of this invention are characterizedby the following features:

(A) High breaking stress and impact resistance;

(B) High tensile strength;

(C) The treated portions of the major surfaces, edges, and sides of theglass article are in compression for a thickness of at least 1 micron(usually below 5 microns) with a maximum compressive stress at thesurface layers of at least 20,000 p.s.i.; I

(D) A central interior thickness of the glass article of about 80percent to 90 percent of the total thickness of the glass article, saidcentral interior thickness belng in tension with a maximum centraltensile stress not substantially exceeding 200 pounds per square inch;

(E) A ratio of maximum surface compression to maximum center tension ofat least about 100 to 1;

(F) The outer surface weight ratio concentration of potassium to sodiumis in excess of 5 to 1. Moreover, this ratio is at least 100 to 250 ormore times the ratio of potassium to sodium in the center of the glassarticle. This outer surface ratio persists for a depth of about 2microns or more, but rarely exceeds microns;

(G) The ability to be cut without violent breakage or shattering andwithout substantial loss in strength properties;

(H) The edges of the glass have the smoothness characteristic of aseamed or polished edge. This is due to the fact that the glasssubjected to treatment has been previously subjected to edge seaming tosmooth the corners and the upper and lower edges of the glass.(Ponsequently, the compressive stress imparted by the chemical treatmentherein contemplated is transmitted around the corners and also more orless continuously from the upper and lower surfaces around the edges ofthe sides of the sheet; and

(I) The treated glass shows little or no strain pattern when viewed in adirection perpendicular to the ma or surfaces of the glass underpolarized light.

The center tension (tensile stress) measurements for the potassium salttreated articles of this invention is determined by an optical method.The method employed is one usually used for rating the degree of annealin annealed glass samples.

The method consists of a measure of the center tension stress obtainedby viewing along the center plane of the 16 glass plate being measured.A graduated quartz switch is used to measure this stress optically,expressed in millimicrous of birefringence (often abbreviated as m perinch or per centimeter of viewing section. The quartz switch can be usedas an accessory on a Polarizing Microscope for viewing small samples orit can be adapted with separate lens, light source and polarizer forviewing through larger samples. It is not practical to view through morethan 20 times the thickness of fiat glass. With the use of prisms, it ispossible to view across the corners of large fiat plates. When measuringcenter tensile stress in curved plates, it is desirable to measure in adirection where the bow is at a minimum or reduce the viewing sectionuntil the bow is less than 10 percent of the glass thickness.

The optical rating of stress expressed per m per square inch can beconverted readily to mechanical pounds per square inch stress units bymultiplying by the stress-optical coetfecient which is 2.13 for mostconventional plate and sheet glass.

The surface compression (compressive stress) measurements are alsodetermined optically, but by a different method.

Surface compression stress is measured with an instrument known as adifferential surface refractometer since it measures a quantityproportional to the differential between refractive indices for lightpolarized parallel and perpendicular to the glass surface. Theinstrument consists essentially of an incandescent light bulb, arectangular prism with a refractive index higher than that of thesurface of the glass, and the viewing telescope. The lower face of theprism is placed in contact with the glass surface with a liquid ofintermediate refractive index between. In the telescope, bright linesare seen and the distance between them is proportional to the surfacestress. This distance is measured with a micrometer eyepiece previouslycalibrated in terms of pounds per square inch per scale division. Byobserving line separation on glass surfaces subjected to knownmechanical stress or convention thermally tempered surfaces usually onlytwo lines are visible, one polarized perpendicular to and the otherparallel to the plane of incidence of the light rays on the glasssurface. The potassium salt treated glass surfaces usually exhibitmultiple sets of lines, each representing the stress at a particulardepth in the stress layer. These lines are usually seen as dispersedspectra due to chromatic variation of the critical angle between theprism and the glass surface. By means of an adjusting screw, the anglebetween the prism and the glass surfaces is adjusted to achromatize thesets of lines representing the stress condition at the surface of theion exchange layer.

One of the prime facets of the articles of the present invention residesin glass closures, which can be transparent (primarily viewingclosures), translucent, and/or opaque. These glass closures can be inthe form of monolithic sheets, e.g., architectural spandrels, orlaminates, e.g., safety-glass Windshields for automobile, airplanes,boats, and other transportation compartments. Architectural laminatesfor buildings and rooms can also be produced readily and are within thepurview of this invention.

The present invention also encompasses articles having only a selectedportions(s) of a respective surface(s) potassium treated, as well aspotassium treatment of the entire surface of all surfaces of thearticle, respectively.

Moreover, this invention includes composite articles containing variouscombinations of types of potassium treated articles, such as compositeinsulation spandrels and like glass closures, whether comprisingindividual monolithic potassium treated sheets or laminates formedtherefrom whether flat or curved. For example, this invention includesinsulation spandrels (two or more potassium treated spandrels mounted ina common frame with air space(s) therebetween) having a decorativelyprinted or colored plastic sheet(s) located in the air space(s). In

such a structure any one of the glass spandrel panels can betransparent, translucent or opaque, and colored or not colored. Forexample, the outer potassium treated spandrel can be transparent and theinner (facing towards the inside of the building) potassium treatedspandrel can be opaque with the intermediately located decorativeplastic sheet located in the air space therebetween. Thus, thedecorative sheet can be seen when viewing from the outside-in but notseen when viewing from the inside-out. Or a reverse arrangement can beadopted. These and other arrangements can be secured with laminatesalso.

Moreover, the articles of the present invention can also be subjected toconventional processing techniques such as thermal tempering (preferablyprior to potassium salt treatment); cutting operations; laminatingoperations; etc., to produce glass articles having enhanced surfacestrength, impact resistance, and penetration resistance.

While the production of the glass articles of this invention has beendescribed hereinabove with regard to the use of high temperatures forfairly short potassium treatment periods, i.e., less than one hour; itshould be realized that potassium treatment can be conducted atsignificantly lower temperatures over extended time periods. Althoughcommercial production favors the short time-high temperature procedureas a matter of economics and convenience; the articles of the presentinvention can be produced at lower temperatures as long as thetemperature is sufiiciently high to allow some potassium exchange totake place, whether using molten or finely divided solid potassium saltsto effect treatment. For example, where time is not a controlling factorthe potassium treatment can be performed at temperatures ranging fromabout 400800 F. over time periods ranging from about several weeks ordays to several hours, with the longer time periods coinciding with thelower temperatures and vice versa.

As noted above, the present invention is by no means limited toproducing glass sheets to be employed in windshields, but also extendsto a wide spectrum of usage, including, but not limited to, productionof glass articles to be used for: building materials; architecturalapplications; bottles, e.g., containers for liquids; drinking glasses;viewing closures such as window panes; safetyglass and other laminatedstructures; glass insulation structures wherein a plurality of glasssheets are arranged in a spaced fashion with a layer of air serving asthe insulation medium; television safety-glass implosion and/orexplosion shields; glass roofs or transparent domes in vehicles andbuildings; experimental devices such as glass engine parts which mustwithstand a high compression;

ceramic and siliceous articles used in the dental arts such as denturesand crown caps; ceramic mufllers for automobiles, airplanes, and othervehicles; etc.

The unusual strength properties which can be achieved by the presentinvention can even be enhanced when glass which has been previouslythermally tempered is treated with a potassium salt.

Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended thatsuch details serve as limitations upon the scope and spirit of thepresent invention. The present invention in its broadest aspects is notnecessarily limited to the specific compositions, temperatures, andcontact times specified in the foregoing examples.

I claim:

l. A laminated windshield comprising two outer sheets of glass, at leastone of which is chemically tempered glass, adhered together by an innerplastic sheet, said chemically tempered sheet comprising asoda-potassiumsilica glass having a compressively stressed surfacehaving a potassium content which is substantially higher than thepotassium content in a tensilely stressed interior of said chemicallytempered glass sheet; a sodium content in the tensilely stressedinterior which is substantially greater than the sodium content in thecompressively stressed surface; said compressive stress at the surfaceextending inwardly from the surface for at least one micron in thicknesstowards the tensilely stressed interior of said glass sheet.

2. The laminated windshield of claim 1. wherein the windshield iscurved.

3. The laminated windshield of claim 1 wherein both outer sheets ofglass are chemically tempered.

4. A glass windshield according to claim 1 in which the ratio of maximumsurface compressive stress to maximum interior tensile stress is atleast 100 to 1.

5. The windshield of claim 1 wherein the plastic inner layer ispolyvinyl butyral.

6. The windshield of claim 1 wherein the thickness of a glass sheet isnot greater than about /a inch.

References Cited UNITED STATES PATENTS 2,198,733 4/1940 Leibig et al65-30 2,779,136 1/1957 Hood et a1 653O 2,924,485 2/1960 Miles 1561023,218,220 11/1965 Weber l6l-l 2,946.711 7/1960 Bragaw et al 161-1993,027,288 3/1962 Keslar 156106 3,282,772 11/1966 Davis 161-465 OTHERREFERENCES M. E. Nordberg et al.: Strengthening by Ion Exchange, J.American Ceramic Soc., 47, No. 5 (May 1964), pp. 215-219.

J. Am. Cer. Soc., February 1962, pp. 59-68, article by S. S. Kistlerentitled, Stresses in Glass Produced by Nonuniform Exchange ofMonovalent Ions.

ROBERT F. BURNETT, Primary Examiner.

W. I. VAN BALEN, Assistant Examiner.

